He doesn't have "red leg".
What people often call red leg isn't even a disease but something that has caused the blood to flow to the capillaries in/on the legs (in other words a symptom) causing a pinkish or reddish tint to show. This often causes a person who hasn't seen real cases of "red leg" to misdiagnose the disease potentially killing the frog through stress and treatments. (In real "red leg" the capillaries have ruptured leading to large amounts of blood in the tissues which causes the red/brown coloration, very different than what you are seeing).
In any case, "red leg" is a potential symptom and really should not be automatically considered a disease as depending on the species of frog/toad in question, redding of the ventral surfaces can be due to a irritant, stress (minor and major), thermal issues, septicemia, fungal infections, viral infections, and protozoal infections.
In fact what is often considered "red leg", a septicemia potentially due to multiple infectious agents but most commonly Aeromonas hydrophilia often does not have this symptom....
I would suggest asking a vet to do a skin scraping to see if there is something causing the reddening of the skin (such as chytrid).
Some comments,
Ed
